Effects of growth and tissue type on the kinetics of 13C and 15N incorporation in a rapidly growing ectotherm

The use of stable isotopes to investigate animal diets, habitat use, and trophic level requires understanding the rate at which animals incorporate the C-13 and N-15 from their diets and the factors that determine the magnitude of the difference in isotopic composition between the animal's diet and that of its tissues. We determined the contribution of growth and catabolic turnover to the rate of C-13 and N-15 incorporation into several tissues that can be sampled non-invasively (skin, scute, whole blood, red blood cells, and plasma solutes) in two age classes of a rapidly growing ectotherm (loggerhead turtles, Caretta caretta). We found significant differences in C and N incorporation rates and isotopic discrimination factors (Delta C-13 = delta C-13(tissues) - delta C-13(diet) and Delta N-15 = delta(15) N-tissues -delta N-15(diet)) among tissues and between age classes. Growth explained from 26 to 100% of the total rate of incorporation in hatchling turtles and from 15 to 52% of the total rate of incorporation in juvenile turtles. Because growth contributed significantly to the rate of isotopic incorporation, variation in rates among tissues was lower than reported in previous studies. The contribution of growth can homogenize the rate of isotopic incorporation and limit the application of stable isotopes to identify dietary changes at contrasting time scales and to determine the timing of diet shifts. The isotopic discrimination factor of nitrogen ranged from -0.64 to 1.77 parts per thousand in the turtles' tissues. These values are lower than the commonly assumed average 3.4 parts per thousand discrimination factors reported for whole body and muscle isotopic analyses. The increasing reliance on non-invasive and non-destructive sampling in animal isotopic ecology requires that we recognize and understand why different tissues differ in isotopic discrimination factors.